The present invention relates to a redispersible material, particularly in powder form, a process for its production and its use, as well as an aqueous system containing the redispersible material.
It is known from the prior art to stabilize aqueous polymer dispersions (latices), which do not in themselves represent thermodynamically stable systems, by dispersants, such as protective colloids or emulsifiers. Apart from the stabilized, aqueous polymer dispersions, interest is also attached to powders obtainable by drying which, due to their characteristics, such as easy handling, easy storage and transportation, as well as easy dosability, are gaining increasing significance.
Such systems can be adapted to the necessary final applications by the targeted choice of functional groups, which for this purpose can be introduced into the polymers used. Numerous such functional groups are known. Due co their reactions, increasing significance is being attached to polymer compositions containing organosilicon groups. For example, EP 187 040 A2 describes a modified polyolefin, which contains a silane or alkoxysilane in copolymerized, grafted or chemically bound form, which is in turn bound to polyvinyl alcohol. The modified polyolefin is used in multilayer films and as an emulsion polymer in polymer blends. A similar product is described by WO 85/4891, i.e. a modified alcohol in the form of a NAD stabilizer (non aqueous dispersant, generally a polyol, which contains unsaturated groups and which is used for stabilizing a further polymer, in that it prevents its agglomeration in steric manner), said alcohol resulting from the reaction of an alcohol with an olefinically unsaturated silicon-containing compound. Thus, with the silane are formed Sixe2x80x94Oxe2x80x94C-bonds, which are not reversibly soluble. Thus, the silanes are exclusively used as coupling agents, the characteristics of the free silane groups playing no part. A preparation of redispersible powders with the polymers described is not possible and not intended.
Redispersible powders based on polymers containing silicon groups are also known from the prior art. For example, EP 754 737 B describes coating compositions, based on a redispersible powder, which contains water-soluble polymer and organosilicon compounds with a specific viscosity and a specific molecular weight. The coating composition is used on metal, wood, plastics and mineral substrates, such as cement concrete, natural stone, sandy limestone, etc. DE 195 26 759 relates to crosslinkable powders redispersible in water, based on ethylenically unsaturated monomers, such as vinyl esters with 1 to 18 carbon atoms, optionally branched alkyl carboxylic acids, (meth)acrylic esters with 1 to 18 carbon atoms, optionally branched alcohols, olefins, dienes, vinyl aromatics and/or vinyl halides, as well as 0.05 to 15.0 wt. % of one or more unsaturated silicon compounds, based on the total monomer mixture.
According to EP 228 657 B1 redispersible powders containing at least one organic silicon compound are prepared, which contain at least 30 wt. % organic silicon compound and at least 50 wt. % have a boiling point of at least 150xc2x0 C. at 1020 hPa, on average max 1.8 SiC-bound, organic groups per silicon atom and per molecule at least one Sixe2x80x94Hxe2x80x94, Sixe2x80x94OHxe2x80x94 or at least one Sixe2x80x94Oxe2x80x94R-group, in which R can be substituted by an alkoxy group, as well as 5 to 50 wt. % water-soluble polymers, based on the total weight of the organic silicon compound. The powder is prepared by spray drying in known manner. Prior to spraying further additives can be added, such as solvents, emulsifiers, antifoaming agents, dyes, pigments, fillers, etc. A reversible reaction with the silicon-containing groups is not described.
WO 95/20626 describes water-redispersible dispersion powder compositions based on water-insoluble polymers and one or more organosilicon compounds. Preparation takes place by the emulsion polymerization of one or more monomers in the presence of one or more water-dispersible organosilicon compounds and spray drying the products obtained, optionally before or after the addition of additives, such as protective colloids and anti-blocking agents. As suitable protective colloids are mentioned polyvinyl alcohols, their derivatives, polysaccharides in water-soluble form, such as starch, cellulose, proteins, synthetic polymers, etc. and as antiblocking agents calcium or magnesium carbonate, talc, gypsum, silica and silicates. The protective colloid in the form of polyvinyl alcohol is only optionally added. Spray drying can alternatively take place before or after polyvinyl alcohol addition and the polyvinyl alcohol exclusively fulfils the function of a protective colloid. The prerequisites for a reaction of the silicon-containing group with the polyvinyl alcohol, accompanied by the formation of a reversibly soluble bond do not exist and this is also not intended.
WO 95/20627 of Wacker-Chemie GmbH with the same application date as WO 95/20626 describes substantially the same redispersible powders. However, the aqueous organopolymer dispersion and the aqueous dispersion of the silicon compound are either jointly sprayed and dried or separately sprayed and jointly dried. Here again, prior to spraying and drying, adjuvants can be optionally added.
As is known, in the aforementioned organosilicon polymers, no matter whether they are redispersible powders or in some other administration form, the silicon groups present should react with inorganic substrates, such as glass, wood, sand; cement-containing materials or similar substrates. Copolymerizable oxysilanes, as are typically present in a polymer, under conventional reaction conditions hydrolyze within minutes to the reactive silanols. This is represented by the following reaction equation (1):
polymer-Sixe2x80x94OR+H2Oxe2x86x92polymer-Sixe2x80x94OH+ROHxe2x80x83xe2x80x83(1) 
As inorganic SiO2 containing materials, such as cement, mortar and glass, to a certain extent have latent Sixe2x80x94OH-groups, silane-modified latices can form covalent, irreversible bonds with the inorganic material. This leads to improved characteristics with respect to adhesion or flexibility, i.e. the setting behaviour and processability of the end products. Such a reaction of a polymer containing silanol groups with an inorganic matrix, which has latent Sixe2x80x94OH-groups, is reproduced by the following reaction equation (2):
polymer-Sixe2x80x94OH+HOxe2x80x94Si-matrixxe2x86x92polymer-Sixe2x80x94Oxe2x80x94Si-matrix+H2Oxe2x80x83xe2x80x83(2) 
It is directly clear from reaction equation (2), that the polymers containing the silanol groups also react with one another in the aqueous system and, accompanied by dehydration the following reaction equation (3) occurs:
polymer-Sixe2x80x94OH+HOxe2x80x94Si-polymerxe2x86x92polymer-Sixe2x80x94Oxe2x80x94Si-polymer+H2Oxe2x80x83xe2x80x83(3) 
Equation (3) represents an undesired secondary reaction of the silanol groups, which competes with the intended reaction according to reaction equation (2). No soluble bonds are formed, so that the silanol groups obtained lose their activity and are no more available for the reaction. Thus, the polymer particles are irreversibly crosslinked by the formation of Sixe2x80x94Oxe2x80x94Si-bonds. In order to prevent such a reaction, i.e. an irreversible bond formation, in the prior art either no Sixe2x80x94OH-groups are used in the polymer material or in planned manner compounds are selected, which cannot enter into such reactions and which are consequently hydrolysis-stable under the reaction conditions. However, such silicon group-containing polymers have for the final applications a generally significantly reduced reactivity (cf. reaction equation (2)), so that more aggressive reaction conditions are needed or the desired coupling reactions of the silanols with the matrix materials, such as with cement-containing products and the like, only occur to an inadequate extent.
This becomes more important when transforming the polymer materials into redispersible powders where the above-described secondary reaction (reaction equation (3)) becomes the main reaction during the preparation of redispersible powders by removing water, e.g. by spray drying
Therefore the problem of the invention is to make available polymers containing silanol to functionalities, in which the silanol groups in aqueous systems are not to be deactivated by the formation of irreversible bonds and consequently have advantageous or, in certain cases, improved characteristics with respect to their final uses, such as e.g. in plastic-containing, cementbound systems and as a result of the desired following reactions lead to improved use products. In addition, a high concentration of such silanol groups in the polymer must be possible. In addition, both aqueous dispersions and redispersible powders must be obtainable from the silanol group-containing polymers, without there being a deactivation following redispersion. The choice of polymer starting materials must also be of a flexible nature.
According to the invention the above problem is solved by a redispersible material, particularly in powder form, which comprises a polymer containing silanol groups, which are provided with a protective group.
Thus, by protecting the silanol groups contained in a polymer a premature deactivation by irreversible bond formation is prevented. The protective group blocks the active centres in the form of silanol groups temporarily against the attack of reagents, such as e.g. further silanol groups, so that reactions such as oxidation, reduction, substitution, condensation, etc. cannot take place. Such functional groups made temporarily non-reactive by a protective group are also known as xe2x80x9clatentxe2x80x9d groups.